A known method for patterning a first layer of a material overlying a second layer of material is photolithography. A common problem occurs in the practice of conventional photolithography when a structure is formed of a first layer of material and overlies a second layer of material having similar etch rates, and only the first layer is desired to be etched. Under these circumstances, overetching of the second layer of the structure is difficult to avoid since there is no selectivity between the first and second layers of material. In other words, if the two materials have the same etch rate, it is difficult to determine the precise moment in time that the pattern in the first material has been completely formed and the etch can be terminated without substantially etching into the surface of the underlying second layer of material. Overetching of the second layer of material causes the exposed surface of the second layer to become recessed into the bulk of the second layer of material. Recession of the surface of the second layer of material occurs in areas unmasked by the pattern formed in the first layer of material.
Photolithography is widely used to pattern a variety of structures and devices comprising both lines (pattern features) and spaces (pattern windows). The problems of surface recession can be found equally among pattern lines and pattern spaces. Topographical recessions, in the surface of certain manufactured structures such as optical masks, can cause serious degradation of performance. For instance, a wide range of semiconductor structures such as bipolar emitters, and buried contacts have recessed regions which can cause surface electrical leakage problems. The leakage problem affects many types of semiconductor devices formed from materials such as silicon, gallium arsenide, and other well known materials in the semiconductor industry.
A method known to overcome the aforementioned disadvantage of conventional photolithography is the method using an etch stop layer. The etch stop layer is a layer of material different from the first and second layers of material and, therefore, has a different etch rate. The etch stop layer is typically deposited over the second layer of material and underlies the first layer to be etched. Consequently, a high selectivity between the etch stop layer and the first layer of material has been provided. A disadvantage of the etch stop layer is that some of the etch stop layer material remains intact under areas masked by the pattern in the first layer. The presence of this material can negate some of the usefulness of the resulting structure.
In the manufacture of modern bipolar transistors and BiCMOS structures, it is common practice to pattern a layer of polysilicon, using conventional methods, to simultaneously form an emitter structure of a bipolar transistor and a gate structure of an MOS transistor. A known disadvantage of polysilicon emitters, formed using conventional photolithographic methods, is the recession of the silicon substrate underlying the polysilicon layer in the extrinsic or intrinsic base region. The recession is caused by overetching the silicon substrate and is due to the problem of overetch previously mentioned. Unfortunately, the recessed silicon surface can adversely affect the electrical performance of semiconductor devices, such as bipolar transistors. The recessive surface in bipolar transistors, for example, can cause excessive, peripheral base current leakage and beta (transistor gain) degradation. In BiCMOS structures, the bipolar transistor exhibits leakage characteristics due to the presence of a recessed silicon surface. The electrical leakage can severely degrade the electrical performance of BiCMOS circuits which are designed to have high performance and low power consumption.